This invention relates generally to a platform load cell wherein a transducer is supported within a parallelogram guiding frame which conveys forces to be measured to the transducer and, more particularly, to a shift-error adjuster for compensating the guiding frame for off-center loading.
There are many applications for platform load cells with a primary example being the retail scale which is used, for example, in a supermarket, for weighing meat, produce and the like to determine prices for commodities to be sold. In such weighing scales, a weight or force to be measured is transmitted by a guiding frame, which may be referred to herein as a flexure, to a transducer, typically comprising strain gauges, which in turn generate signals representative of the weight or force.
A common form of platform load cell comprises a parallelogram guiding frame or flexure which includes force transmitting beams to which strain gauges are directly connected or to which a load cell including strain gauges may be coupled. Illustrative parallelogram flexures are disclosed, for example, in U.S. Pat. Nos. 4,181,011 and 4,196,784. Unfortunately, problems have been encountered when using such flexures.
One particularly annoying problem with parallelogram flexures is that the weight indicated by a weighing scale varies dependent upon the position that an object to be weighed is placed on the weighing platform of the scale. This problem of shift errors in the registered weights due to off-center loading of a scale and compensation for such shift errors is addressed in Tramposch et al., U.S. Pat. No. 4,485,881. Tramposch et al. discloses a parallelogram flexure for supporting a transducer wherein shift-error adjusting means is included within a fixed first section of the flexure.
The adjusting means comprises a generally U-shaped opening formed into the inner side of the upper end of the first section of the flexure. A vertical adjusting mechanism comprising a preload screw extends downwardly through the center of the U-shaped opening. A horizontal adjusting mechanism comprises two wedges, one on either side of the preload screw, which are formed to engage the inside walls of the U-shaped opening and may be drawn thereinto by means of adjustment screws threaded into the wedges and extending through the closed base of the opening. By adjusting the preload screw or simultaneously adjusting both adjustment screws, the vertical spacing between flexjoints adjacent to the first fixed section of the flexure may be adjusted, while the vertical spacing between the front or the rear of the two flexjoints may be separately adjusted by adjusting one or the other of the adjustment screws.
While Tramposch et al. provides shift-error adjustment for a parallelogram flexure of a platform load cell, the adjustment wedges obviously interact with one another during adjustment of the flexure and require additional steps for assembly of the flexure and, in certain applications, it may be inconvenient to have to make both a vertical and a horizontal adjustment. It is, therefore, desirable to have a simplified shift-error adjuster for a parallelogram flexure which may be adjusted along only a single direction. Further, the provision of a variety of flexure shift-error adjusting arrangments also serves to advance the art and such alternatives may be preferred in certain applications.